JSLint Expected '===' and instead saw '=='

javascript jslint
Metropolis picture Metropolis · Sep 17, 2010 · Viewed 60.1k times · Source

Recently I was running some of my code through JSLint when I came up with this error. The thing I think is funny about this error though is that it automatically assumes that all == should be ===.

Does that really make any sense? I could see a lot of instances that you would not want to compare type, and I am worried that this could actually cause problems.

The word "Expected" would imply that this should be done EVERY time.....That is what does not make sense to me.

Answer

Christian C. Salvadó picture Christian C. Salvadó · Sep 17, 2010

IMO, blindly using ===, without trying to understand how type conversion works doesn't make much sense.

The primary fear about the Equals operator == is that the comparison rules depending on the types compared can make the operator non-transitive, for example, if:

A == B AND
B == C

Doesn't really guarantees that:

A == C

For example:

'0' == 0;   // true
 0  == '';  // true
'0' == '';  // false

The Strict Equals operator === is not really necessary when you compare values of the same type, the most common example:

if (typeof foo == "function") {
  //..
}

We compare the result of the typeof operator, which is always a string, with a string literal...

Or when you know the type coercion rules, for example, check if something is null or undefinedsomething:

if (foo == null) {
  // foo is null or undefined
}

// Vs. the following non-sense version:

if (foo === null || typeof foo === "undefined") {
  // foo is null or undefined
}

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